Apparatus for controlling a power-assisted steering gear in response to vehicle speed

ABSTRACT

An apparatus ( 10 ) for helping to turn steerable wheels ( 12 ) of a vehicle comprises a hydraulic power-assisted steering gear ( 16 ). An electric motor ( 92 ) actuates the steering gear ( 16 ). A pump ( 84 ) supplies the steering gear ( 16 ) with hydraulic fluid. A vehicle speed sensor ( 102 ) senses vehicle speed and provides a vehicle speed signal. A controller ( 104 ) is responsive to the vehicle speed signal for controlling the pump ( 84 ). The controller ( 104 ) activates the pump ( 84 ) to supply hydraulic fluid to the steering gear ( 16 ) when the vehicle speed signal indicates a vehicle speed below a predetermined value and the controller ( 104 ) deactivating the pump ( 84 ) when the vehicle speed signal indicates a vehicle speed above the predetermined value.

RELATED APPLICATIONS

This application is a divisional of co-pending patent application Ser.No. 10/397,613, filed Mar. 26, 2003, now U.S. Pat. No. 6,966,398.

TECHNICAL FIELD

The present invention relates to an apparatus for controlling apower-assisted steering gear in response to vehicle speed.

BACKGROUND OF THE INVENTION

A conventional hydraulic power-assisted steering system includes asteering gear having a hydraulic motor. A fluid pump draws hydraulicfluid from a fluid reservoir and supplies the hydraulic fluid to thesteering gear. Typically, the engine of the vehicle powers the pump tosupply hydraulic fluid from a fluid reservoir to the steering gear. Thesteering gear includes a control valve. The control valve is responsiveto steering inputs for directing hydraulic fluid to the hydraulic motor.The hydraulic motor is operatively connected to the steerable wheels ofthe vehicle and, when actuated, helps to turn the steerable wheels.

In the conventional hydraulic power-assisted steering system, hydraulicfluid is continually circulating between the pump, the control valve,and the fluid reservoir. As the speed of the vehicle increases, the needfor power-assisted steering decreases. The conventional hydraulicpower-assisted steering system operates independently of the vehiclespeed.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus for helping to turnsteerable wheels of a vehicle. The apparatus comprises a hydraulicpower-assisted steering gear. An electric motor actuates the steeringgear. A pump supplies the steering gear with hydraulic fluid. A vehiclespeed sensor senses vehicle speed and provides a vehicle speed signal. Acontroller is responsive to the vehicle speed signal for controlling thepump. The controller activates the pump to supply hydraulic fluid to thesteering gear when the vehicle speed signal indicates a vehicle speedbelow a predetermined value and the controller deactivating the pumpwhen the vehicle speed signal indicates a vehicle speed above thepredetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of an apparatus constructed inaccordance with the present invention;

FIG. 2 is a process diagram of a control process for the apparatus ofFIG. 1;

FIG. 3 is a schematic illustration of an apparatus constructed inaccordance with a second embodiment of the present invention;

FIG. 4 is a schematic illustration of an apparatus constructed inaccordance with a third embodiment of the present invention;

FIG. 5 is a process diagram of a control process for the apparatus ofFIG. 4;

FIG. 6 is a schematic illustration of an apparatus constructed inaccordance with a fourth embodiment of the present invention; and

FIG. 7 is a schematic illustration of an apparatus constructed inaccordance with a fifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an apparatus 10 constructed in accordance with thepresent invention. The apparatus 10 helps to turn steerable wheels 12 ofa vehicle in response to rotation of a hand wheel 14 of the vehicle.

The apparatus 10 includes a hydraulic power assisted steering gear 16.The steering gear 16 illustrated in FIG. 1 is an integral hydraulicpower assisted steering gear. Other hydraulic power assisted steeringgears are contemplated by this invention, for example, the steering gearmay be a rack and pinion steering gear.

The steering gear 16 includes a housing 18 and a drive mechanism 20. Thedrive mechanism 20 is moved in response to rotation of the hand wheel 14of the vehicle. The motion of the drive mechanism 20 results in aturning of the steerable wheels 12 of the vehicle.

The drive mechanism 20 includes a sector gear 22 having a plurality ofteeth 24. The sector gear 22 is fixed on an output shaft 26 that extendsoutwardly through an opening in the housing 18. The output shaft 26 istypically connected to a pitman arm (not shown) that is connected to thesteering linkage of the vehicle. The dashed lines between the outputshaft 26 and the steerable wheels 12 in FIG. 1 schematically representthe pitman arm and steering linkage. Thus, as the sector gear 22rotates, the output shaft 26 is rotated to operate the steering linkage.As a result, the steerable wheels 12 of the vehicle are turned.

The steering gear 16 further includes a hydraulic motor 28 for movingthe drive mechanism 20. The hydraulic motor 28 is located within thehousing 18 of the steering gear 16. The housing 18 of the steering gear16 has an inner cylindrical surface 30 defining a chamber 32. A piston34 is located within the chamber 32 and divides the chamber intoopposite chamber portions 36 and 38. One chamber portion 38 is locatedon a first side of the piston 34 and the other chamber portion 38 islocated on a second opposite side of the piston. The piston 34 creates aseal between the respective chamber portions 36 and 38 and is capable ofaxial movement within the chamber 32. This axial movement of the piston34 results in an increase in volume of one chamber portion, e.g., 36,and a corresponding decrease in volume of the other chamber portion,e.g., 38.

A series of rack teeth 40 is formed on the periphery of the piston 34.The rack teeth 40 act as an output for the hydraulic motor 28 and meshwith the teeth 24 formed on the sector gear 22 of the drive mechanism20.

A control valve 44 directs the flow of hydraulic fluid to the hydraulicmotor 28. The control valve 44 is located within the housing 18 of thesteering gear 16. An inlet 46 provides fluid communication to thecontrol valve 44 and an outlet 52 provides fluid communication away fromthe control valve.

The control valve 44 is of a conventional design and includes a valvecore portion 54 and a valve sleeve portion 56 that are connectedtogether through a torsion bar 58. The control valve 44 directs fluid toan appropriate chamber portion 36 or 38 of the hydraulic motor 28. Theflow of hydraulic fluid toward one of the chamber portions 36 or 38increases the pressure within that chamber portion. When the pressure ofone chamber portion, e.g., 36, increases relative to the pressure of theother chamber portion, e.g., 38, the piston 34 moves axially and thevolume of the higher-pressure chamber portion increases. The volume ofthe higher-pressure chamber portion, e.g., 36, increases until thepressure within the chamber portions 36 and 38 equalizes.

As the volume of one chamber portion, e.g., 36, increases, the volume ofthe other chamber portion, e.g., 38, decreases. The decreasing chamberportion, e.g., 38, is vented to allow a portion of the fluid containedin the decreasing chamber portion to escape. The escaping fluid exitsthe housing 18 of the steering gear 16 via the outlet 52.

The piston 34 of the hydraulic motor 28 contains a bore 72, partiallyshown in FIG. 1, which is open toward the control valve 44. The valvesleeve portion 56 and a follow-up member 74 collectively form anintegral one-piece unit that is supported for rotation relative to thepiston 34 by a plurality of balls 76. The outer periphery 78 of thefollow-up member 74 is threaded. The plurality of balls 76 interconnectsthe threaded outer periphery 78 of the follow-up member 74 with aninternal thread 80 formed in the bore 72 of the piston 34. As a resultof the interconnecting plurality of balls 76, axial movement of thepiston 34 causes the follow-up member 74 and the valve sleeve portion 56to rotate. The rotation of the follow-up member 74 and the valve sleeveportion 56 returns the control valve 44 to the neutral position.

The valve core portion 54 of the control valve 44 is fixedly connectedto an input shaft 82. As shown schematically by dashed lines in FIG. 1,the input shaft 82 is fixedly connected to the hand wheel 14 of thevehicle. Rotation of the hand wheel 14 results in rotation of the inputshaft 82 and rotation of the valve core 52.

The torsion bar 84 of the steering gear 16 has first and second ends 84and 86, respectively. The first end 84 of the torsion bar 58 is fixedrelative to the input shaft 82 and the valve core portion 54 of thecontrol valve 44. The second end 86 of the torsion bar 58 is fixedrelative to the valve sleeve portion 56 of the control valve 44 and thefollow-up member 74.

When the resistance to turning of the steerable wheels 12 of the vehicleis below a predetermined amount, rotation of the hand wheel 14 istransferred through the torsion bar 58 and causes rotation of thefollow-up member 74. As a result, the control valve 44 remains in theneutral position. Rotation of the follow-up member 74 causes movement ofthe piston 34 and results in turning of the steerable wheels 12.

When resistance to turning the steerable wheels 12 of the vehicle is ator above the predetermined amount, rotation of the follow-up member 74is resisted. As a result, rotation of the hand wheel 14 rotates thefirst end 84 of the torsion bar 58 relative to the second end 86 of thetorsion bar. The rotation of the first end 84 of the torsion bar 58relative to the second end 86 of the torsion bar results in torsion ortwisting across the torsion bar. As a result of torsion across thetorsion bar 58, the valve core portion 54 of the control valve 44rotates relative to the valve sleeve portion 56 of the control valve andthe control valve 44, when supplied with hydraulic fluid, directs fluidtoward one of the chamber portions 36 or 38 of the hydraulic motor 28.

As discussed above, when fluid is directed toward one of the chamberportions 36 or 38, the piston 34 moves within the chamber 32. Movementof the piston 34 results in turning of the steerable wheels 12 of thevehicle, as well as, rotation of the follow-up member 74. As discussedabove, rotation of the follow-up member 74 rotates the valve sleeveportion 56 until the control valve 44 is again in the neutral position.When the control valve 44 is in the neutral position, the torsion acrossthe torsion bar 58 is removed and the first end 84 of the torsion bar isno longer rotated or twisted relative to the second end 86 of thetorsion bar.

The apparatus 10 includes a pump 84 that is in fluid communication withthe steering gear 16 for supplying hydraulic fluid to the steering gear.The pump 84 draws hydraulic fluid from a fluid reservoir 86 and suppliesthe hydraulic fluid to the inlet 46 of the steering gear 16. The pump 84may be of any conventional design. An electric pump motor 88 isoperatively connected with the pump 84. The pump motor 88 receiveselectric power from a power source 90. The power source 90 may be thevehicle battery. When energized, the pump motor 88 drives the pump 84 tosupply hydraulic fluid to the steering gear 16.

The apparatus 10 also includes an electric motor 92 for actuating thesteering gear 16. The electric motor 92 may be of any conventionaldesign. The electric motor 92 receives electric power from the powersource 90. An output shaft (not shown) of the electric motor 92 isconnected to the input shaft 82 of the steering gear 16. Preferably, agear assembly 94 is used to connect the output shaft of the electricmotor 92 to the input shaft 82 of the steering gear 16. When theelectric motor 92 receives electric power, the output shaft of theelectric motor 92 rotates the input shaft 82 of the steering gear 16.Thus, the electric motor 92 is said to be “in series connection” withthe hydraulic motor 28.

The apparatus 10 also includes a column torque sensor 96 for sensingcolumn torque and outputting a signal indicative of the column torque.Column torque is related to the torsion across the torsion bar 58 andthe material properties of the torsion bar. The column torque sensor 96may measure the rotational movement of the first end 84 of the torsionbar 58 relative to the second end 86 of the torsion bar. The movement ofthe valve core portion 54 relative to the valve sleeve portion 56alternatively may be measured for indicating the relative rotationbetween the first end 84 and the second end 86 of the torsion bar 58.

The apparatus 10 also includes a plurality of vehicle condition sensors98, 100 and 102 and a controller 104. Preferably, the vehicle conditionsensors include a lateral acceleration sensor 98, a hand wheel rotationsensor 100, and a vehicle speed sensor 102. Each sensor 98, 100 and 102is electrically connected to the controller 104.

The lateral acceleration sensor 98 senses the lateral acceleration ofthe vehicle and generates an electrical signal indicative of the sensedlateral acceleration. The hand wheel rotation sensor 100 senses themagnitude, rate, and acceleration of rotation of the vehicle hand wheel14 and generates electrical signals indicative of these parameters. Thehand wheel rotation magnitude is the angle of rotation of the hand wheel14 relative to a straight ahead position of the hand wheel. Rotation ofthe hand wheel 14 in a first direction may be designated as a positivevalue and rotation of the hand wheel 14 in a second direction, oppositethe first direction, may be designated as a negative value. The handwheel rotation sensor 100, or the controller 104, may determine the rateof rotation of the hand wheel 14 by taking a time differential of themagnitude and may determine the hand wheel acceleration by taking a timedifferential of the rate of rotation. The vehicle speed sensor 102senses the vehicle speed and generates an electrical signal indicativeof the speed.

The controller 104 receives the signals generated by the lateralacceleration sensor 98, the hand wheel rotation sensor 100, and thevehicle speed sensor 102. Additionally, the controller 104 receives thecolumn torque signal from the column torque sensor 96. The controller104 analyzes the respective signals using a known algorithm andgenerates a control signal for controlling the electric motor 92. Theelectric motor 92 is controlled for actuating the steering gear 16 so asto provide a predetermined resistance to rotation of the hand wheel 14.

When the electric motor 92 receives electric power, the output shaft ofthe electric motor 92, through the gear assembly 94, rotates the inputshaft 82 of the steering gear 16. As a result, the electric motor 92assists the operator in rotating the input shaft 82 of the steering gear18.

Additionally, the controller 104 compares the signal from the vehiclespeed sensor 102 to a predetermined value. When the comparison indicatesthat the vehicle speed is below the predetermined value, the controller104 actives the pump 84 to supply hydraulic fluid to the steering gear16. For example, when the vehicle speed is below twenty kilometers perhour, the controller 104 activates the pump 84. To activate the pump 84of FIG. 1, the controller 104 controls the pump motor 88 to drive pump84. When the controller 104 determines that the vehicle speed is greaterthan the predetermined value, the controller 104 deactivates the pump 84to discontinue supplying fluid to the steering gear 16. To deactivatethe pump 84, the controller 104 shuts down the pump motor 88. Thus, whenthe controller 104 determines that the vehicle speed is greater than thepredetermined value, the controller 104 terminates operation of thehydraulic motor 28 of the steering gear 16.

FIG. 2 is a flow chart illustrating the process 200 performed by thecontroller 104 of FIG. 1. The process 200 begins at step 202. At step204, the controller 104 monitors the handwheel rotation. At step 206,the controller 104 monitors the vehicle speed. At step 208, thecontroller 104 monitors the lateral acceleration of the vehicle and, thecolumn torque is monitored at step 210. At step 212, the controller 104analyzes the signals received at steps 204, 206, 208, and 210 and outputthe control signal to control the electric motor 92. At step 214, adetermination is made as to whether the vehicle speed is below apredetermined value. If the determination at step 214 is negative, theprocess 200 returns to step 204. If the determination at step 214 isaffirmative, the process 200 proceeds to step 216. At step 216, thecontroller 104 activates the pump 84 to supply fluid to the steeringgear 16. From step 216, the process 200 returns to step 204.

The apparatus 10 of the present invention provides a power-assistedsteering system that includes a hydraulic boost when the vehicle isoperating at low speeds. Generally, at lower vehicle speeds, theresistance to rotation of steerable wheels 12 is increased and themagnitude of a turn of the steerable wheels 12 is increased, as comparedto while operating a higher vehicle speeds. The apparatus 10 of thepresent invention aids the vehicle driver in turning the steering wheels12 while operating at the lower speeds in which additional steeringassistance is generally most helpful.

FIG. 3 illustrates an apparatus 10A constructed in accordance with asecond embodiment of the present invention. Structures of FIG. 3 thatare the same as or similar to structures of FIG. 1 are numbered usingthe same reference numbers and are not discussed in detail with regardto FIG. 3. Only the differences between the apparatus 10 of FIG. 1 andthe apparatus 10A of FIG. 3 are discussed in detail below.

Unlike the pump 84 of the apparatus 10 illustrated in FIG. 1, anelectric pump motor 88 does not drive the pump 110 of the apparatus 10Aof FIG. 3. The pump 110 of FIG. 3 is operatively connected to the engine112 of the vehicle and is driven by the engine 112 of the vehicle. Aclutch mechanism 114 is interposed between the vehicle engine 112 andthe pump 110. The clutch mechanism 114 is actuatable between an engagedcondition and a disengaged condition. In the engaged condition, theclutch mechanism 114 mechanically interconnects or couples the vehicleengine 112 and the pump 110 for driving the pump to supply hydraulicfluid to the steering gear 16. In the disengaged condition, the vehicleengine 112 and the pump 110 are not mechanically interconnected, i.e.,are decoupled from one another, so that no power is transferred betweenthe vehicle engine 112 and the pump 110. Thus, when the clutch mechanism114 in the disengaged condition, the pump 110 is deactivated anddiscontinues supplying hydraulic fluid to the steering gear 16.

The clutch mechanism 114 is operatively connected to the controller 104and the controller actuates the clutch mechanism 114 between the engagedand disengaged conditions. When the controller 104 determines that thevehicle speed is below the predetermined value (e.g., 20 kph), thecontroller 104 actuates the clutch mechanism 114 into the engagedcondition. When the clutch mechanism 114 is actuated into the engagedcondition, the pump 110 is activated to supply hydraulic fluid to thesteering gear 16. When the controller 104 determines that the vehiclespeed is greater than the predetermined value, the controller 104actuates the clutch mechanism 114 into the disengaged condition. Whenthe clutch mechanism 114 is in the disengaged condition, the pump 110 isdeactivated and discontinues supplying fluid to the steering gear 16.Thus, when the controller 104 determines that the vehicle speed isgreater than the predetermined value, the controller 104 terminatesoperation of the hydraulic motor 28 of the steering gear 16.

In a manner similar to the apparatus 10 of FIG. 1, the apparatus 10A ofFIG. 3 provides a hydraulic boost when the vehicle is operating at lowspeeds. Thus, the apparatus 10A of FIG. 3 aids the vehicle driver inturning the steering wheels 12 while operating at the lower speeds inwhich additional steering assistance is generally most helpful.

FIG. 4 illustrates an apparatus 10B constructed in accordance with athird embodiment of the present invention. Structures of FIG. 4 that arethe same as or similar to structures of FIG. 1 are numbered using thesame reference numbers and are not discussed in detail with regard toFIG. 4. Only the differences between the apparatus 10 of FIG. 1 and theapparatus 10B of FIG. 4 are discussed in detail below.

The apparatus 10B of FIG. 4 includes a motor current sensor 118 forsensing the actual current of the electric motor 92 and for providing amotor current signal indicative of the sensed current. The motor currentsensor 118 is operatively connected to the controller 104. Thecontroller 104 receives the motor current signal from the motor currentsensor 118 and uses the motor current signal to determine the outputtorque of the electric motor 92.

Additionally, the apparatus 10B of FIG. 4 includes first and secondfluid supply systems 120 and 122, respectively, each for supplyinghydraulic fluid to the steering gear 16. The first fluid supply system120 includes a first pump 124 and a first reservoir 126. The first pump124 draws hydraulic fluid from the first reservoir 126. A first electricpump motor 128 is operatively connected with the first pump 124. Whenenergized, the first pump motor 128 drives the first pump 124 to supplyhydraulic fluid to the steering gear 16. Hydraulic fluid returned fromthe steering gear 16 to the first fluid supply system 120 is directedinto the first reservoir 126.

The second fluid supply system 122 includes a second pump 130 and asecond reservoir 132. The second pump 130 draws hydraulic fluid from thesecond reservoir 132. A second electric pump motor 134 is operativelyconnected with the second pump 130. When energized, the second pumpmotor 134 drives the second pump 130 to supply hydraulic fluid to thesteering gear 16. Hydraulic fluid returned from the steering gear 16 tothe second fluid supply system 122 is directed into the second reservoir132.

The apparatus 10B of FIG. 4 also includes a two-position valve assembly138. The two-position valve assembly 138 is interposed between the firstand second pumps 124 and 130 and the inlet 46 and the steering gear 16.The two-position valve assembly also is interposed between the outlet 52of the steering gear 16 and the first and second reservoirs 126 and 132.

In a first position, illustrated in FIG. 4, the two-position valveassembly 138 places the first fluid supply system 120 in fluidcommunication with the steering gear 16. Thus, when in the firstposition, the two-position valve places the first pump 124 and the firstreservoir 126 in fluid communication with the steering gear 16. Also,when the two-position valve assembly 138 is in the first position, thesecond fluid supply system 122 is isolated from fluid communication withthe steering gear 16.

In a second position, the two-position valve assembly 138 places thesecond fluid supply system 122 in fluid communication with the steeringgear 16. Thus, when in the second position, the two-position valveassembly 138 places the second pump 130 and the second reservoir 132 influid communication with the steering gear 16. Also, when thetwo-position valve assembly 138 is in the second position, the firstfluid supply system 120 is isolated from fluid communication with thesteering gear 16.

A valve drive 140 is connected with the two-position valve assembly 138for positioning of the two-position valve assembly. Preferably, thevalve drive 140 is a solenoid. The valve drive 140 is operativelyconnected to the controller 104. The controller 104 controls the valvedrive 140. Thus, the controller 104 controls the position of thetwo-position valve assembly 138 between the first and second positionsfor selectively placing either the first or second fluid supply system120 or 122 in fluid communication with the steering gear 16.

FIG. 5 is a flow chart illustrating the process 500 performed by thecontroller 104 of FIG. 5. The process 500 begins at step 502. At step504, the controller 104 monitors the hand wheel 14 rotation. At step506, the controller 104 monitors the vehicle speed. At step 508, thecontroller 104 monitors the lateral acceleration of the vehicle and, thecolumn torque is monitored at step 510. At step 512, the controller 104analyzes the signals received at steps 504, 506, 508, and 510 andoutputs the control signal to control the electric motor 92. Theelectric motor 92 is controlled to provide a predetermined resistance torotation of the hand wheel 14. At step 514, a determination is made asto whether the vehicle speed is below a first predetermined value X. Ifthe determination at step 514 is negative, the process 500 returns tostep 504. If the determination at step 514 is affirmative, the process500 proceeds to step 516.

At step 516, the controller 104 activates the first pump 124 to supplyfluid to the control valve 44 of the steering gear 16. From step 516,the process 500 proceeds to step 518. At step 518, the controller 104actuates the two-position valve assembly 32 into the first position sothat the first pump 124 and the first reservoir 126 are in fluidcommunication with the steering gear 16, as shown in FIG. 4.

At step 520, the controller 104 monitors the motor torque of theelectric motor 92. The process 500 then proceeds to step 522 in which adetermination is made as to whether the motor torque from step 520 isgreater then a second predetermined value Y. If the determination atstep 522 is negative, the controller 104 assumes that the apparatus 10Bis properly operating and the process 500 returns to step 504. If thedetermination at step 522 is affirmative and the motor torque is abovethe second predetermined value Y, the controller 104 assumes that thehydraulic motor 28 of the steering gear 16 is not responding properly toactuation of the steering gear 16 by the electric motor 92. From step522, the process 500 proceeds to step 524. At step 524, the controller104 activates the second pump 130 to supply fluid to the control valve44 of the steering gear 16. Also, at step 524, the controller 104 shutsdown the first pump 124. From step 524, the process 500 proceeds to step526. At step 526, the controller actuates the two-position valveassembly 138 into the second position so that the second pump 130 andthe second reservoir 132 are in fluid communication with the steeringgear 16. Thus, in response to motor torque above the secondpredetermined value Y, the controller 104 places the second fluid supplysource 122 in fluid communication with the steering gear 16 and isolatesthe first fluid supply source 120 from fluid communication with thesteering gear 16.

As illustrated in the process 500 of FIG. 5, the apparatus 10B of FIG. 4includes redundant first and second fluid supply systems 120 and 122,each including a pump 124 and 130, respectively, and a reservoir 126 and132, respectively. The controller 104 of the apparatus 10B generallyactivates the first pump and places the first fluid supply system 120 influid communication with the steering gear 16 when the vehicle speed isbelow a first predetermined value X. If the first pump 124 is activatedand the first fluid supply system 120 is in fluid communication with thesteering gear 16 and a determination is made that the motor torque isgreater than the second predetermined value Y, the controller 104 of theapparatus 10B shuts down the first pump 124, activates the second pump130, and places the second fluid supply system 122 in fluidcommunication with the steering gear 16. Thus, the apparatus 10B of thepresent invention provides redundant systems 120 and 122 for supplyinghydraulic fluid to the control valve 44 of the steering gear 16.

FIG. 6 illustrates an apparatus 10C constructed in accordance with afourth embodiment of the present invention. Structures of FIG. 6 thatare the same as or similar to structures of FIG. 4 are numbered usingthe same reference numbers and are not discussed in detail with regardto FIG. 6. Only the differences between the apparatus 10B of FIG. 4 andthe apparatus 10C of FIG. 6 are discussed in detail below. FIG. 6 onlyillustrates a portion of the steering gear 16, including the fluid inlet46 and outlet 52 of the steering gear 16. It is to be understood thatthe steering gear 16 of the apparatus 10C of FIG. 6 is identical to thesteering gear 16 of FIG. 4.

The apparatus 10C of FIG. 6 also includes first and second fluid supplysystems 120 and 122, respectively, each of which is shown schematicallyin FIG. 6. The apparatus 10C of FIG. 6 also includes two separate anddistinct two-position valve assemblies, a supply valve 146 and a returnvalve 148.

The supply valve 146 includes parallel inlets 150 and parallel outlets152. The first and second pumps 124 and 130, respectively, are connectedto the inlets 150 of the supply valve 146, each pump to a respectiveinlet. One of the outlets 152 of the supply valve 146 is connected tothe inlet 46 of the steering gear 16 and the other outlet 152 isconnected to an inlet 156 of the return valve 148.

The supply valve 146 has first and second positions. In a firstposition, shown in FIG. 6, the supply valve 146 places the first pump124 in fluid communication with the inlet 46 of the steering gear 16 andplaces the second pump 130 in fluid communication with the return valve148. In the second position, the supply valve 146 places the second pump130 in fluid communication with the inlet 46 of the steering gear 16 andplaces the first pump 124 in fluid communication with the return valve148.

A valve drive 154 is connected to the supply valve 146 for actuating thesupply valve between the first and second positions. The valve drive 154is preferably a solenoid. The valve drive 154 is operatively connectedto the controller 104 and the controller controls actuation of the valvedrive 154.

The return valve 148 also includes parallel inlets 156 and paralleloutlets 158. An outlet 152 of the supply valve 146 is connected to oneof the inlets 156 of the return valve 148 and the outlet 52 of thesteering gear 16 is connected to the other inlet 156 of the return valve148. One of the outlets 158 of the return valve 148 is connected to thefirst reservoir 126 and the other outlet 158 is connected to the secondreservoir 132.

The return valve 148 has first and second positions. In a firstposition, shown in FIG. 6, the return valve 148 connects the outlet 52of the steering gear 16 to the first reservoir 126 and connects theoutlet 152 of the supply valve 146 to the second reservoir 132. In thesecond position, the return valve 148 connects the outlet 52 of thesteering gear 16 to the second reservoir 132 and connects the outlet 152of the supply valve 146 to the first reservoir 126.

A valve drive 160 is connected to the return valve 148 for actuating thereturn valve 148 between the first and second positions. The valve drive160 is preferably a solenoid. The valve drive 160 is operativelyconnected to the controller 104 and the controller controls actuation ofthe valve drive.

The apparatus 10C of FIG. 6 is controlled in a manner similar to theapparatus 10B of FIG. 4. When the controller 104 determines that thevehicle speed is below the first predetermined value X, the controller104 activates the first pump 124 and places the first fluid supplysource 120 in fluid communication with the steering gear 16. If thecontroller 104 determines that the motor torque of electric motor 92 isabove the second predetermined value Y while the first pump 124 isactivated and in fluid communication with the steering gear 16, thecontroller 104 shuts down the first pump 124, activates the second pump130, and actuates the supply and return valves 146 and 148 to place thesecond fluid supply source 122 in fluid communication with the steeringgear 16.

FIG. 7 illustrates an apparatus 10D constructed in accordance with afifth embodiment of the present invention. Structures of FIG. 7 that arethe same as or similar to structures of FIG. 6 are numbered using thesame reference numbers and are not discussed in detail with regard toFIG. 7. Only the differences between the apparatus 10C of FIG. 6 and theapparatus 10D of FIG. 7 are discussed in detail below. Similar to FIG.6, FIG. 7 only illustrates a portion of the steering gear 16, includingthe fluid inlet 46 and outlet 52 of the steering gear 16.

The apparatus 10D of FIG. 7 also includes first and second fluid supplysystems 120 and 122, respectively, each of which is shown schematicallyin FIG. 7. The apparatus 10D of FIG. 7 also includes three separate anddistinct two-position valve assemblies, first and second supply valves170 and 712, respectively, and a return valve 174.

The first supply valve 170 controls fluid flow from the first fluidsupply system 120 and includes a single inlet 176 and two outlets 178.The first pump 124 is connected to the inlet 176 of the first supplyvalve 170. One of the outlets 178 of the first supply valve 170 isconnected to the inlet 46 of the steering gear 16, through a check valve180, and the other outlet 178 of the first supply valve 170 is connectedto the first reservoir 126.

The first supply valve 170 has first and second positions. In a firstposition, shown in FIG. 7, the first supply valve 170 places the firstpump 124 in fluid communication with the inlet 46 of the steering gear16. In the second position, the first supply valve 170 places the firstpump 124 in fluid communication with the first reservoir 126.

A valve drive 182 is connected to the first supply valve 170 foractuating the first supply valve 170 between the first and secondpositions. The valve drive 182 is preferably a solenoid. The valve drive182 is operatively connected to the controller 104 and the controllercontrols actuation of the valve drive 182.

The second supply valve 172 controls fluid flow from the second fluidsupply system 122 and includes a single inlet 184 and two outlets 186.The second pump 130 is connected to the inlet 184 of the second supplyvalve 172. One of the outlets 186 of the second supply valve 172 isconnected to the inlet 46 of the steering gear 16, through a check valve188, and the other outlet 186 of the second supply valve 172 isconnected to the second reservoir 132.

The second supply valve 172 has first and second positions. In a firstposition, shown in FIG. 7, the second supply valve 172 places the secondpump 130 in fluid communication with the second reservoir 132. In thesecond position, the second supply valve 172 places the second pump 130in fluid communication with the inlet 46 of the steering gear 16.

A valve drive 190 is connected to the second supply valve 172 foractuating the second supply valve between the first and secondpositions. The valve drive 190 is preferably a solenoid. The valve drive190 is operatively connected to the controller 104 and the controllercontrols actuation of the valve drive 190.

The return valve 194 also includes a single inlet 192 and two outlets194. The outlet 52 of the steering gear 16 is connected to the inlet 192of the return valve 174. One of the outlets 194 of the return valve 174is connected to the first reservoir 126 and the other outlet 194 isconnected to the second reservoir 132.

The return valve 174 has first and second positions. In a firstposition, shown in FIG. 7, the return valve 174 places the outlet 52 ofthe steering gear 16 in fluid communication with the first reservoir126. In the second position, the return valve 174 places the outlet 52of the steering gear 16 in fluid communication with the second reservoir132.

A valve drive 196 is connected to the return valve 174 for actuating thereturn valve between the first and second positions. The valve drive 196is preferably a solenoid. The valve drive 196 is operatively connectedto the controller 104 and the controller controls actuation of the valvedrive.

The apparatus 10D of FIG. 7 is controlled in a manner similar to theapparatuses 10B and 10C of FIGS. 4 and 6, respectively. When thecontroller 104 determines that the vehicle speed is below the firstpredetermined value X, the controller 104 activates the first pump 124and places the first fluid system 120 in fluid communication with thesteering gear 16, i.e., places the first pump 124 in fluid communicationwith the inlet 46 of the steering gear 16 and places the outlet 52 ofthe steering gear 16 in fluid communication with the first reservoir126. If the controller 104 determines that the motor torque of electricmotor 92 is above the second predetermined value while the first pump124 is activated and the first fluid system 120 is in fluidcommunication with the steering gear 16, the controller 104 shuts downthe first pump 124, activates the second pump 130, and places the secondfluid supply system 122 in fluid communication with the steering gear16, i.e., places the second pump 130 in fluid communication with theinlet 46 of the steering gear 16 and places the outlet 52 of thesteering gear 16 in fluid communication with the second reservoir 132.

Similar to the apparatuses 10 and 10A of FIGS. 1 and 3, the apparatuses10B, 10C, and 10D of FIGS. 4, 6, and 7 provide a power assisted steeringsystem that includes a hydraulic boost when the vehicle is operating atlow speeds. The apparatuses 10B, 10C, and 10D of FIGS. 4, 6, and 7 alsoinclude redundant fluid supply systems 120 and 122 for supplyinghydraulic fluid to a hydraulic motor 28 of a steering gear 16 forproviding the hydraulic boost.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. Suchimprovements, changes and modifications within the skill of the art areintended to be covered by the appended claims.

1. An apparatus for helping to turn steerable wheels of a vehicle, theapparatus comprising: a hydraulic power-assisted steering gear; anelectric motor for actuating the steering gear; a pump for supplying thesteering gear with hydraulic fluid; at least one valve assemblyinterposed between the pump and the steering gear; a vehicle speedsensor for sensing vehicle speed and for providing a vehicle speedsignal; and a controller responsive to the vehicle speed signal, thecontroller actuating the at least one valve assembly to selectivelyplace the pump in fluid communication with the steering gear to providehydraulic fluid to the steering gear when the vehicle speed signalindicates a vehicle speed below a predetermined value and the controlleractuating the at least one valve assembly to selectively place the pumpout of fluid communication with the steering gear when the vehicle speedsignal indicates a vehicle speed above the predetermined value.
 2. Theapparatus of claim 1 wherein the electric motor is responsive torotation of a hand wheel of the vehicle for actuating the steering gear,the controller controlling the electric motor to provide a predeterminedresistance to rotation of the hand wheel.
 3. The apparatus of claim 1wherein the pump is a first pump, the first pump being in fluidcommunication with a first reservoir, the first pump and first reservoirforming a first fluid supply source for supplying fluid to the steeringgear through the at least one valve assembly, the apparatus furtherincluding a second fluid supply source for supplying fluid to thesteering gear, the controller controlling activation and deactivation ofboth the first and second fluid supply sources.
 4. The apparatus ofclaim 3 wherein the electric motor is responsive to rotation of a handwheel of the vehicle for actuating the steering gear, the controllercontrolling the electric motor to provide a predetermined resistance torotation of the hand wheel, a current sensor for monitoring current ofthe electric motor and for providing a current signal to the controller,the controller being responsive to the current signal for controllingactivation and deactivation of the first and second fluid supplysources.
 5. The apparatus of claim 3 wherein the at least one valveassembly includes a supply valve for placing one of the first and secondpumps in fluid communication with an inlet of the steering gear and areturn valve for placing one of the first and second reservoirs in fluidcommunication with an outlet of the steering gear.
 6. The apparatus ofclaim 3 wherein the at least one valve assembly includes first andsecond supply valves and a return valve, the first supply valve beingactuatable for placing the first pump in fluid communication with aninlet of the steering gear, the second supply valve being actuatable forplacing the second pump in fluid communication with an inlet of thesteering gear, and the return valve for placing one of the first andsecond reservoirs in fluid communication with an outlet of the steeringgear, the controller controlling the return valve for placing the firstreservoir in fluid communication with the outlet of the steering gearwhen the first supply valve places the first pump in fluid communicationwith the inlet of the steering gear and for placing the second reservoirin fluid communication with the outlet of the steering gear when thesecond supply valve places the first pump in fluid communication withthe inlet of the steering gear.